Method of stabilising clay or shale

ABSTRACT

A method of reducing the swelling of shale or clay encountered in a wellbore, the method comprising introducing into the wellbore a composition comprising: (a) a continuous aqueous phase; (b) a source of borate ions; (c) a source of ions selected from the group consisting of alkali metal ions, alkaline earth metal ions and ammonium ions; and (d) optionally a source of at least one sugar selected from the group consisting of monosaccharides and oligosaccharides having from  2  to  4  saccharide groups.

The present invention relates to a composition useful for stabilizing a clayey or shaley formation surrounding a wellbore. In particular, the present invention relates to a clay or shale swelling inhibitor composition for use during drilling, completing or maintaining wellbores.

Drilling fluids are used in the drilling of oil and gas wells. In rotary drilling operations drilling fluids are pumped down from the surface through a drill string to a drill bit. They emerge through ports in the drill bit and return to the surface via an annular space located between the drill string and the walls of the borehole. The functions of drilling fluids may be multiple: for example, they serve to cool and lubricate both the drill bit and drill string, they transport drill cuttings to the surface, they equalize the pressure between the fluids in the wellbore and the formation fluids, they prevent “squeezing” of the wellbore or caving of the formation into the wellbore, or minimise any potential damage to the “pay zone” of the wellbore.

Drilling fluids are of two basic types, oil-based (hereinafter referred to as OBMs—oil based muds), and water-based (hereinafter referred to as WBMs—water based muds). OBMs are superior in performance to WBMs in several important respects including lubricating properties and thermal stability thereby allowing wells to be drilled at a faster rate than when using WBMs resulting in considerable cost savings. In particular, OBMs are useful where the downhole temperature is high, for example when drilling deviated wells through high temperature formations. Furthermore, OBMs mitigate problems associated with swelling and dispersion of clays or shales, which are frequently encountered during drilling with WBMs. Such problems will hereinafter be referred to as “clay or shale destabilization”.

Unfortunately, OBMs are less attractive than WBMs from an environmental perspective. The disposal of spent OBMs, and the associated problems of cuttings clean-up and disposal are posing increasing difficulties for the oil and gas exploration industry. One proposal has been the use of biodegradable oils to formulate OBMs, but such products may not comply with future legislation which is expected to impose more stringent limits on disposal of waste materials. The consequence of this is that much effort has been expended in the development of WBMs having improved performance in terms of reducing clay or shale destabilization. If no attempts are made to inhibit hydration and swelling of clays or shales the consequences can be severe. Thus, clay or shale destabilization may result in weaknesses developing in the formation, possibly leading to erosion of the borehole. Also, as would be well known to the person skilled in the art, the phenomenon of “stuck pipe” can occur, and furthermore, logging operations during drilling can be hampered.

A number of approaches have been proposed for reducing the clay or shale destabilization characteristics of WBMs. The use of salts such as Group IA metal salts, in particular, potassium chloride, to balance the water activity between the clay or shale and the drilling fluid, or even to provide an osmotic gradient that leads to a net flow of water out of the clay or shale, has been employed to prevent clay or shale hydration. This approach may be combined with the use of silicates which are capable of forming osmotic membranes on the exposed surface of the clay or shale, as described, for example, in U.S. Pat. No. 3,640,343, and van Oort et. al. in SPE/IADC paper No. 35059, presented at the LADC/SPE Drilling Conf. New Orleans, 12-15 Mar. 1996. The osmotic membrane facilitates the flow of water out of the clay or shale whilst inhibiting the diffusion of ions between the clay or shale and the fluids in the wellbore thereby improving the clay or shale stabilization characteristics of the WBM.

The precipitation of silicates on the exposed surface of the clay or shale is also believed to produce a physical barrier against invasion of fluids from the wellbore into the clay or shale. The use of silicates, however, may be associated with problems of high drilling torques and poor lubricity owing to the tendency of silicates to precipitate out on metal surfaces, for example, on the drill bit. Also, the use of silicates may lead to problems of incompatibility with conventional drilling fluid additives. Finally, silicate solutions are highly alkaline and this can lead to difficulties with safe storage and handling.

Other proposals for improving the clay or shale stabilization performance of WBMs include the addition of glycols or polyols. Suitable glycols or polyols include, for example, polyglycerols, glycols, polyalkylene glycols (PAG), e.g., polyethylene glycols (PEG), polypropylene glycols (PPG) and copolymers of ethylene and propylene glycols, alcohol ethoxylates and glycol ethers as described in for example, EP 0495579, U.S. Pat. No. 4,830,765, U.S. Pat. No. 4,172,800 and The Society of Petroleum Engineers Reports SPE 25989 and 28818. In The Society of Petroleum Engineers Report SPE 28960 it is disclosed that potassium ions work synergistically with glycols in drilling fluids to improve the shale stabilization performance of the WBMs.

Other proposals described in the art to improve the clay or shale stabilization properties of WBMs include the use of additives such as aluminium complexes, chemically modified starch, chemically modified cellulosic materials, water soluble polyacrylamides and other water soluble polymers, lime or gypsum, asphaltene derived products and calcium lignosulphonates.

The use of borates in hydraulic fracturing fluids is described in U.S. Pat. No. 5372732, U.S. Pat. No. 5445223, GB 2253868 and WO 87/00236 while U.S. Pat. No. 5220960 discloses the use of borates as cement retardants for completion operations.

CA 1248337 teaches the use of borates in the field for low specific gravity non-damaging workover and completion fluids. CA 1303841 describes the use of borates in water profile control in oil recovery. E L Bigelow in The Society of Petroleum Engineers publication SPE 27644 discloses that borates maybe used in pulsed neutron logging. Borates have also been used in lost circulation treatments (fluid loss pills) during drilling operations (as described in U.S. Pat. No. 5,372,732).

U.S. Pat. No. 6,105,691 refers to the use of boric acid or glycerol-borate esters in drilling fluids for the purposes of improving the lubricity of the drilling fluid and its clay dispersion characteristics. However, U.S. Pat. No. 6,105,691 is silent concerning any beneficial effect of borate on the clay or shale stabilizing properties of drilling fluids.

RU 1699991 relates to a silicate drilling mud having increased mud stability and calcium chloride resistance. The drilling mud is prepared by adding boric acid and sodium or potassium silicate to water to produce a gel-like mass, which is then diluted until the required viscosity, structural and mechanical properties have been obtained. An organic stabilizer may then be added to adjust filtration. There is no suggestion that a sugar may be added to the mud to increase its shale or clay stabilization characteristics.

An object of the present invention is to provide an aqueous based composition, in particular, an aqueous based drilling fluid composition, having improved clay or shale stabilization characteristics. A further object of the present invention is to provide a method of reducing the swelling of shale or clay encountered in a wellbore, for example, during drilling through a formation.

It has now been found that an aqueous based composition which contains a combination of (a) borate ions and (b) ions selected from the group consisting of alkali(ne earth) metal ions and ammonium ions exhibits markedly improved clay or shale stabilization properties compared with aqueous based compositions containing components (a) or (b) alone. It has also been found that the addition of a silicate to the aqueous based composition containing components (a) and (b) has a detrimental effect on its clay or shale stabilization properties. It has further been found that the addition of at least one sugar, selected from the group consisting of monosaccharides and oligosaccharides having from 2 to 4 saccharide groups, to the aqueous based composition containing components (a) and (b) improves the clay or shale stabilization characteristics thereof.

Thus, in a first embodiment of the present invention there is provided a use of a combination of (a) a source of borate ions, (b) a source of ions selected from the group consisting of alkali metal ions, alkaline earth metal ions and ammonium ions and (c) optionally a source of at least one sugar selected from the group consisting of monosaccharides and oligosaccharides having from 2 to 4 saccharide groups, in a composition comprising a continuous aqueous phase, to improve the clay or shale stabilizing properties thereof.

In a second embodiment of the present invention there is provided a method of reducing the swelling of shale or clay encountered in a wellbore, the method comprising introd ucing into the wellbore a composition comprising:

-   (a) a continuous aqueous phase; -   (b) a source of borate ions; -   (c) a source of ions selected from the group consisting of alkali     metal ions, alkaline earth metal ions and ammonium ions; and -   (d) optionally a source of at least one sugar selected from the     group consisting of monosaccharides and oligosaccharides having from     2 to 4 saccharide groups.

The composition that is introduced into the wellbore may be used in drilling, completing or maintaining a wellbore. Where the composition is used in drilling a wellbore, the composition is preferably circulated in the wellbore thereby stabilising the clay or shale.

According to a preferred aspect of the present invention there is provided a method of reducing the swelling of shale or clay encountered during the drilling of a wellbore through a formation using a drill string disposed within the wellbore, said drill string having a first end and a second end, the first end of the drill string being located at or near the surface of the wellbore and the second end of the drill string being in communication with a drill bit having ports therein, wherein:

-   (A) a drilling fluid composition comprising (a) a continuous aqueous     phase, (b) a source of borate ions, (c) a source of ions selected     from the group consisting of alkali metal ions, alkaline earth metal     ions and ammonium ions, and (d) optionally a source of at least one     sugar selected from the group consisting of monosaccharides and     oligosaccharidcs having from 2 to 4 saccharide groups is introduced     into the first end of the drill string, is pumped through the drill     string from the first end to the second end thereof and is     discharged into the wellbore through the ports in the drill bit; and -   (B) the drilling fluid composition is recycled to the first end of     the drill string via an annular space which is provided between the     drill string and the walls of the wellbore.

Suitably, the continuous aqueous phase of the composition may be fresh water, tap water, sea water, river water or aquifer water.

Preferably, the source of borate ions is an alkali metal borate, an ammonium borate or mixtures thereof. Preferably, the source of the borate ions is a borate of generic formula (1): n(M¹ ₂O)m(B₂O₃).xH₂O   (I) wherein M¹ represents an alkali metal (preferably, sodium or potassium) or NH4, n is an integer in the range 0 to 5, m is an integer in the range 1 to 7, the ratio of n:m is 0-1:1 and x is an integer in the range 0 to 10. Regardless of the source of borate ions, it is preferred that the ratio of n:m in the composition is in the range 0.1:1 to 1.2:1. This ratio may be adjusted by addition of an alkali metal hydroxide (for example, sodium hydroxide or potassium hydroxide) or ammonium hydroxide to the composition.

The source of borate ions may be a refined product or a natural mineral. Preferably, the source of borate ions may be selected from, but not limited to, the group consisting of disodium tetraborate pentahydrate (Na₂O.2B₂O₃.5H₂O), disodium tetraborate decahydrate or tincal (Na₂O.2B₂O₃.10H₂O), disodium tetraborate (Na₂O.2B₂O₃), sodium metaborate tetrahydrate (Na₂O.B₂O₃.8H₂O), sodium metaborate dihydrate (Na₂O.B₂O₃.4H₂O), sodium pentaborate pentahydrate (Na₂O.5B₂O₃.10H₂O), disodium octaborate tetrahydrate (Na₂O.4B₂O₃ .4H₂O), boric acid (H₃BO₃), dipotassium tetraborate tetrahydrate (K₂O.2B₂O₃.4H₂O), potassium pentaborate tetrahydrate (K₂O.5B₂O₃.8H₂O), diammonium tetraborate tetrahydrate ((NH₄)₂O.2B₂O₃.4H₂O), ammonium pentaborate tetrahydrate ((NH₄)₂O.5B.₂O₃.8H₂O) and caesium pentaborate tetrahydrate (Cs₂O.5B₂O₃.8H₂O).

Where the source of borate ions is an alkali metal borate or an ammonium borate, the borate will also act as a source of alkalimetal ions or of ammonium ions respectively. If necessary, a source of additional alkali metal ions or ammonium ions, may be added to the composition, as described above.

The amount of borate, expressed as B₂O₃ in lb/bbl (pounds per barrel) present in the continuous aqueous phase of the composition is preferably in the range 0.1 to 150 lb/bbl, and more preferably, in the range 0.5 to 50 lb/bbl.

It is envisaged that the source of borate ions may be sold as a concentrate ready for dilution with the composition. The concentrate may comprise a solution or dispersion of a borate in an aqueous liquid or a non aqueous liquid. Suitable aqueous liquids are as described above. Suitable non aqueous liquids include polar solvents such as alcohols and glycols. Preferably, the concentrate comprises a solution or dispersion of a borate in an aqueous liquid. The amount of borate, expressed as B₂O₃ in lb/bbl (pounds per barrel) present in the concentrate is preferably in the range 10 to 250 lb/bbl.

Suitably, the source of the ions selected from the group consisting of alkali metal ions, alkaline earth metal ions and ammonium ions is a water soluble salt of an alkali(ne earth) metal, a water soluble ammonium salt or mixtures thereof. It is preferred that the water soluble salt of the alkali(ne earth) metal is not a silicate. Preferably the water soluble salt is a potassium salt, caesium salt or ammonium salt including, but not limited to, potassium bromide, caesium bromide, ammonium bromide, potassium chloride, caesium chloride, ammonium chloride, potassium hydroxide, caesium hydroxide and ammonium hydroxide. For example, if boric acid is used as the source of borate, then the water soluble salt is preferably selected from potassium hydroxide, caesium hydroxide and ammonium hydroxide.

Suitably, the alkali(ne earth) metal salt, ammonium salt or mixtures thereof may be present in the continuous aqueous phase of the composition in an amount in the range 0.1 to 150 lb/bbl, preferably, in the range 0.5 to 100 lb/bbl.

It is envisaged that the source of the alkali(ne earth) metal ions, ammonium ions or mixtures thereof may be sold as a concentrate ready for dilution with the composition. Preferably, the concentrate may comprise a solution or dispersion of an alkali(ne earth) metal salt, ammonium salt or mixtures thereof in an aqueous liquid. Preferably, the concentration of alkali(ne earth) metal salt, ammonium salt or mixtures thereof in the concentrate is in the range 10 to 150 lb/bbl, preferably 10 to 100 lb/bbl.

The sugar may be selected from the group consisting of monosaccharides, and oligosaccharides having 2 to 4 saccharide groups. Preferred monosaccharides include glucose and fructose. Preferred disaccharides include sucrose (for example, obtained from cane or beet), maltose and lactose. It is also envisaged that the source of the sugar may be a mixture of sugars, for example, glucose syrup, golden syrup, molasses or Activ 7™ (a water soluble liquid syrup obtained by partial hydrolysis of starch).

Suitably, the amount of sugar in the continuous aqueous phase of the composition is in the range from 0.1 to 150 lb/bbl, preferably 0.5 to 50 lb/bbl.

It is envisaged that the source of the sugar may be sold as a concentrate ready for dilution with the composition. Preferably, the concentrate may comprise a solution or dispersion of the sugar in an aqueous liquid. Preferably, the concentration of the sugar in the concentrate is in the range 10 to 300 lb/bbl, preferably 20 to 300 lb/bbl.

It is also envisaged that (a) the source of borate ions, (b) the source of the alkali(ne earth) metal ions, ammonium ions or mixtures thereof and (c) the source of sugar may be sold as a mixed concentrate. Suitably, the amount of components (a), (b) and (c) in the mixed concentrate are as described above for the individual concentrates.

An advantage of the present invention is that the composition is stable at elevated temperature for a prolonged period of time. Typically, the composition is stable for a period of at least 12 hours, preferably at least 16 hours at a temperature of at least 100° C., preferably at least 120° C.

A further advantage of the present invention is that the source of borate ions acts as a pH buffer for the composition thereby controlling the pH at a value which is typically above 8, preferably above 9. Accordingly, there is no requirement to include a pH control agent such as sodium hydroxide or potassium hydroxide.

The composition may be beneficially used in combination with conventional additives for improving the clay or shale stabilization properties of a drilling or completion fluid. These conventional additives include, but are not limited to glycols.

The composition may also contain additional ingredients such as weighting agents, e.g., barite, haematite, or galena; viscosifiers, e.g. xanthan gum; fluid loss control agents, e.g., starch or cellulose derivatives (e.g., carboxymethyl cellulose); defoamers and lubricants.

Yet a further advantage of the composition is that it is compatible with conventional fluid loss control agents.

There is no special requirement in relation to the preparation of the composition. The source of borate ions, the source of the alkali(ne earth) metal ions, ammonium ions or mixtures thereof, and optionally the source of the sugar may be added in any order to the continuous aqueous phase (either as concentrates or as solids). Gentle heating is optional to dissolve or disperse the source of borate ions, the source of alkali(ne earth) metal ions, ammonium ions or mixtures thereof, and the optional source of the sugar in the continuous aqueous phase of the composition.

As discussed above, the sugar has been found to improve the clay or shale stabilization properties of the aqueous based composition used in the present invention. Thus, in yet a further embodiment of the present invention there is provided a composition comprising (a) a continuous aqueous phase, (b) a source of borate ions, (c) a source of ions selected from the group consisting of alkali metal ions, alkaline earth metal ions and ammonium ions and (d) a source of at least one sugar selected from the group consisting of monosaccharides and oligosaccharides having from 2 to 4 saccharide groups.

The components (a) to (d) of the composition of the present invention have the preferred features described above.

The performance of the composition of the present invention in clay or shale stabilization is now illustrated by reference to the following examples.

EXAMPLE 1

Aqueous based compositions, were prepared comprising the components shown in Table 1. Each composition was made up to a weight of 80 g with demineralised water and was contained in a 100 ml glass bottle.

A model shale (London Clay; 10 g) screened to a particle size range of 2-4 mm, was added to each bottle. The bottles were then sealed with screw caps and rolled at 20 rpm for 16 hours (bottle rolling test). Each sample mixture was then passed through a separate pre-weighed 500 micron sieve (i.e. a sieve which retains particles having a size of greater than 500 microns). The clay retained on each sieve was rinsed gently with tap water. The sieves plus retained clay were then placed in a drying oven at a temperature of 130° C. for 24 hours.

Each sieve was re-weighed and the percentage of dry clay retained on the sieves was calculated using the following formula, taking into account the original moisture content of the clay as 20 wt. %: % clay retained=12.5×[(weight of sieve+dried retained clay(g))−(weight of clean sieve (g))]

The higher the % clay retained the better the performance of the composition in stabilising the model shale. The results obtained are shown in Table 1. TABLE 1 Results of Bottle Rolling Tests Composition % clay retained 1. 80 g demineralised water (comparative) 0.8 2. 25 lb/bbl KCl (comparative) 9.1 3. 25 lb/bbl NH₄Cl (comparative) 27.5 4. 25 lb/bbl CsCl (comparative) 79.1 5. 25 lb/bbl KCl + 3 wt. % DCP208¹ (comparative) 81.9 6. 25 lb/bbl KCl + 3.47 wt. % Activ 7² (comparative) 33.1 7. 25 lb/bbl LiCl (comparative) −0.9 8. 25 lb/bbl MgCl₂ (comparative) 0.4 9. 25 lb/bbl CaCl₂ (comparative) 0.4 10. 25 lb/bbl KCl + 2.5 wt. % Na₂O.2B₂O₃.5H₂O + 93.6 2.5 wt. % Na₂O.B₂O₃.8H₂O 11. 25 lb/bbl KCl + 10 wt. % Na₂O.B₂O₃.8H₂O 94.0 12. 25 lb/bbl NH₄Cl + 2.5 wt. % Na₂O.2B₂O₃.5H₂O + 93.6 2.5 wt. % Na₂O.B₂O₃.8H₂O 13. 25 lb/bbl CsCl + 2.5 wt. % Na₂O.2B₂O₃.5H₂O + 96.0 2.5 wt. % Na₂O.B₂O₃.8H₂O 14. 25 lb/bbl KCl + 3 wt. % DCP208¹ + 97.6 2.5 wt. % Na₂O.2B₂O₃.5H₂O + 2.5 wt. % Na₂O.B₂O₃.8H₂O 15. 25 lb/bbl KCl + 3.47 wt. % Activ 7² + 97.2 1.51 wt. % Na₂O.2B₂O₃.5H₂O + 1.96 wt. % H₃BO₃ 16. 25 lb/bbl LiCl + 2.5 wt. % Na₂O.2B₂O₃.5H₂O + 77.4 2.5 wt. % Na₂O.B₂O₃.8H₂O 17. 25 lb/bbl MgCl₂ + 2.5 wt. % Na₂O.2B₂O₃.5H₂O + 64.1 2.5 wt. % Na₂O.B₂O₃.8H₂O 18. 25 lb/bbl CaCl₂ + 2.5 wt. % Na₂O.2B₂O₃.5H₂O + 74.1 2.5 wt. % Na₂O.B₂O₃.8H₂O 19. 4.0 wt. % K₂O.2B₂O₃.5H₂O (1.826 wt. % B₂O₃) 80.2 20. 3.4 wt. % (NH₄)₂O.2B₂O₃.5H₂O 68.0 (1.826 wt. % B₂O₃) ¹a glycol supplied by BP Chemicals ²wheat starch by-product (glucose syrup) supplied by Roquette Freres

The unexpectedly superior performance of the compositions comprising a source of borate ions and a source of alkali(ne earth) metal ions or ammonium ions in stabilizing shale (examples 10-20) is evident. The effect of adding a source of sugar (Activ 7) to a composition containing a source of borate ions and a source of alkali(ne earth) metal ions or ammonium ions to improve the shale stabilization performance of the composition still further is evidenced by example 15.

EXAMPLE 2

The procedure in Example 1 was repeated with further compositions to assess the effect of borate ion concentration and the ratio of n:m (i.e. molar ratio of Na₂O:B₂O₃) on shale stabilization performance. The results are given in Table 2. TABLE 2 Results of Bottle Tests to Determine the Effect of Borate Ion Concentration and Ratio of n:m on Shale Stabilization Performance wt. % Composition B₂O₃ n:m % retained 21. 10. 25 lb/bbl KCl + 0.25 wt. % 0.183 0.67:1 59.5 Na₂O.2B₂O₃.5H₂O + 0.25 wt. % Na₂O.B₂O₃.8H₂O 22. 25 lb/bbl KCl + 1.25 wt. % 0.913 0.67:1 96.5 Na₂O.2B₂O₃.5H₂O + 1.25 wt. % Na₂O.B₂O₃.8H₂O 23. 25 lb/bbl KCl + 2.5 wt. % 1.826 0.67:1 96.6 Na₂O.2B₂O₃.5H₂O + 2.5 wt. % Na₂O.B₂O₃.8H₂O 24. 25 lb/bbl KCl + 3.24 wt. % H₃BO₃ 1.826  0.0:1 20.8 25. 25 lb/bbl KCl + 1.53 wt. % 1.826 0.20:1 76.1 Na₂O.2B₂O₃.5H₂O + 1.94 wt. % H₃BO₃ 26. 25 lb/bbl KCl + 3.82 wt. % 1.826 0.50:1 97.8 Na₂O.2B₂O₃.5H₂O 27. 25 lb/bbl KCl + 7.32 wt. % 1.826  1.0:1 97.6 Na₂O.B₂O₃.8H₂O

The results indicate that an acceptable shale stabilization performance can be achieved with compositions having a range of borate ion concentrations and a range of n:m ratios.

EXAMPLE 3

Water based compositions (350 ml), were prepared comprising the components shown in Table 3 below. Each composition was made up using tap water and was prepared in a plastic bottle. To each of the bottles was added 30 g of 2-4 mm size range Oxford clay. The bottles were sealed and rolled at a speed of approximately 30 rpm in an oven set at a temperature of 65° C. for a period of 16 hours. The compositions were then repeatedly passed through a 2 mm sieve. The clay retained on the sieve was collected and oven dried for a minimum of 16 hours. The % clay retained in the test was calculated taking into account, as in Example 1, the original moisture content of the clay. The results are shown in Table 3. TABLE 3 Results of Bottle Rolling Tests using Oxford Clay % clay Composition pH retained 28. 25 lb/bbl KCl (comparative)  9.4 11.5 29. 25 lb/bbl KCl + 3% Glydril MC¹ (comparative)  7.4 49.7 30. 25 lb/bbl KCl + 2.5% Polyseal silicate² 12³ 99.1 (comparative) 31. 25 lb/bbl KCl + 6 wt. % Na₂O.2B₂O₃.5H₂O  9.3 40.2 32. 25 lb/bbl KCl + 3.47 wt. % Activ 7 + 1.51 wt. %  9³ 102.2 Na₂O.2B₂O₃.5H₂O + 1.96 wt. % H₃BO₃ ¹commercial glycol shale inhibitor product ²2:1 SiO₂:Na₂O ³pH adjusted value

EXAMPLE 4

A so-called Hamster Cage Test for evaluating the shale stabilizing performance of the compositions according to the present invention was performed. This test involved preparing 1500 ml of water based drilling fluid compositions as presented in Table 4 (where the aqueous liquid component of the compositions is demineralised water): TABLE 4 Compositions for use in Hamster Cage Test Drilling Fluid A (comparative) Drilling Fluid B Ingredient lb/bbl of ingredient lb/bbl of ingredient KCl 25 25 Polyanionic cellulose¹ 1.5 1.5 Starch² 4.0 4.0 Xanthan gum³ 1.5 1.0 Na₂O.2B₂O₃.5H₂O 0 8.75 Na₂O.B₂O₃.8H₂O 0 8.75 ¹PAC L - supplied by Baroid ²DEXTRID - supplied by Baroid ³Xanvis - supplied by Schlumberger

The viscosities of the drilling fluid compositions prepared in Table 4 were measured at a temperature of 20° C. using a Fann Viscometer (Model 35SA—Baroid Testing Equipment, Texas, USA). The results of the viscosity tests are presented in Table 5. TABLE 5 Viscosities of Drilling Fluid Compositions Drilling Fluid A Fann viscometer readings (comparative) Drilling Fluid B 600 rpm 49 48 300 rpm 34.5 31 200 rpm 28 25 100 rpm 19.5 17  6 rpm 6 5  3 rpm 4.5 4 PV/cP 14.5 17 YP/lb/100 sqrft 20 14 PH 10.0 (adjusted with NaOH) 9.4

These results demonstrate that drilling fluid compositions with desired rheological properties can be formulated containing borate ions. Furthermore, the presence of borate ions enables lower levels of viscosifying polymer to be employed, and ensures that the fluid is well buffered at the correct pH.

Hamster Cage Test

The Hamster Cage Test involved removing the lids from pre-weighed cages (of 1 mm mesh size) and adding approximately 80 g of London Clay (4-8 mm sieved particle size range) to each cage. The lids were replaced on the cages which were then suspended in a first and a second trough previously filled with drilling fluid compositions A and B respectively. The cages were then rotated automatically for 4 hours. At the end of this period, the cages were removed from the apparatus, the lids opened and the cages and retained cuttings rinsed with tap water. They were then dried in an oven for 16 hours at a temperature of 130° C., and finally re-weighed. The formula used in Example 1 was employed to calculate the percentage of clay cuttings retained in the cage. The results are presented in Table 6: TABLE 6 Results of Hamster Cage Tests % Clay Retained Drilling Fluid A (Comparative) 44.0 Drilling Fluid B 73.1

The improved shale stabilizing performance of the drilling fluid composition containing borate ions and potassium ions (Drilling Fluid B) is evident from these results.

EXAMPLE 5

The fluid loss characteristics and viscosity of drilling fluid composition C, comprising the ingredients given in Table 7, were measured before and after high temperature ageing (hot rolling) at a temperature of 1 20° C. for 16 hours. The results are presented in Table 8. TABLE 7 Drilling Fluid Composition C Ingredient lb/bbl of Ingredient KCl 25 Polyanionic cellulose 1.5 Starch 4.0 Xanthan Gum 1.5 Na₂O.2B₂O₃.5H₂O 8.75 Na₂O.B₂O₃.8H₂O 8.75 Barite 100 HMP¹ 10 ¹HMP = Hymod Prima clay obtained from Imerys comprising illite and having a particle size of about 1 to 5 microns. HMP is a non-swelling clay and simulates drilled solids (cuttings) in the formulation.

TABLE 8 Viscosity and Fluid Loss Characteristics of Un-aged and Aged (16 hours at 120° C.) Samples of Drilling Fluid Composition C Fann viscometer readings Un-aged Aged 600 rpm 90 85 300 rpm 62 59 200 rpm 50 47 100 rpm 36 34  6 rpm 12 12  3 rpm 9 10 Gel/10 s 11 14 PV/cP 28 26 YP/lb/100 sqrft 34 33 Fluid loss   1 min 0.0 0.1 7.5 min 0.6 0.9  30 min 2.9 3.7 pH 9.5 9.3

The fluid loss characteristics of drilling fluid composition C are good and this property together with the viscosity of the drilling fluid composition remain substantially unaltered despite high temperature ageing. 

1-20 (cancelled).
 21. A method of reducing the swelling of shale or clay encountered during drilling a wellbore, the method comprising circulating in the wellbore a composition comprising: (a) a continuous aqueous phase; (b) a source of borate ions; (c) a source of ions selected from the group consisting of alkali metal ions, alkaline earth metal ions and ammonium ions; and (d) optionally a source of at least one sugar selected from the group consisting of monosaccharides and oligosaccharides having from 2 to 4 saccharide groups.
 22. A method of reducing the swelling of shale or clay encountered during the drilling of a wellbore through a formation using a drill string disposed within the wellbore, said drill string having a first end and a second end, the first end of the drill string being located at or near the surface of the wellbore and the second end of the drill string being in communication with a drill bit having ports therein, wherein: (A) a drilling fluid composition comprising (a) a continuous aqueous phase, (b) a source of borate ions, (c) a source of ions selected from the group consisting of alkali metal ions, alkaline earth metal ions and ammonium ions, and (d) optionally a source of at least one sugar selected from the group consisting of monosaccharides and oligosaccharides having from 2 to 4 saccharide groups is introduced into the first end of the drill string, is pumped through the drill string from the first end to the second end thereof and is discharged into the wellbore through the ports in the drill bit; and (B) the drilling fluid composition is recycled to the first end of the drill string via an annular space which is provided between the drill string and the walls of the wellbore.
 23. A method according to claim 21 wherein the source of borate ions is selected from the group consisting of sodium borates, potassium borates, caesium borates, ammonium borates, and boric acid.
 24. A method according to claim 23 wherein the source of borate ions is selected from the group consisting of: Disodium tetraborate pentahydrate—Na₂O 2B₂O₃ 5H₂O (borax pentahydrate), Disodium tetraborate decahydrate—Na₂O 2B₂O₃ 10H₂O (borax decahydrate, tincal), Disodium tetraborate tetrahydrate—Na₂O 2B₂O₃ 4H₂O (kernite), Disodium tetraborate—Na₂O 2B₂O₃ (anhydrous borax), Sodium metaborate tetrahydrate—Na₂O B₂O₃8H₂O, Sodium metaborate dihydrate—Na₂O B₂O₃ 4H₂O, Sodium pentaborate pentahydrate—Na₂O 5B₂O₃ 10H₂O, Disodium octaborate tetrahydrate—Na₂O 4B₂O₃ 4H₂O, Boric acid—H₃BO₃, Dipotassium tetraborate tetrahydrate—K₂O 2B₂O₃ 4H₂O, Potassium pentaborate tetrahydrate—K₂O 5B₂O₃ 8H₂O, Diammonium tetraborate tetrahydrate—(NH₄)₂O 2B₂O₃ 4H₂O, Ammonium pentaborate tetrahydrate—(NH₄)₂O 5B₂O₃ 8H₂O, and Caesium pentaborate tetrahydrate—Cs₂O 5B₂O₃ 8H₂O.
 25. A method according to claim 21 wherein the concentration of borate expressed as B₂O₃ in the continuous aqueous phase is in the range 0.1 to 150 lb/bbl.
 26. A method according to claim 21 wherein the source of the alkali metal ions, alkaline earth metal ions and ammonium ions is a water soluble salt of an alkali(ne earth) metal, a water soluble ammonium salt or mixtures thereof.
 27. A method according to claim 26 wherein the salt is selected from the group consisting of potassium salts, caesium salts, and ammonium salts.
 28. A method according to claim 27 wherein the salt is selected from the group consisting of potassium bromide, caesium bromide, ammonium bromide, potassium chloride, caesium chloride, ammonium chloride, potassium hydroxide, caesium hydroxide and ammonium hydroxide.
 29. A method according to claim 26 wherein the concentration of alkali metal salt, alkaline earth metal salt or ammonium salt in the continuous aqueous phase is in the range 0.1 to 150 lb/bbl.
 30. A method according to claim 21 wherein the sugar is selected from the group consisting of glucose, fructose, sucrose, maltose, lactose and mixtures thereof.
 31. A method according to claim 21 wherein the concentration of sugar in the continuous aqueous phase is in the range 0.1 to 150 lb/bbl.
 32. A method according to claim 21 wherein the composition comprises additional additives selected from the group consisting of weighting agents, viscosifiers, fluid loss control agents, defoamers, lubricants, and glycols.
 33. A composition comprising (a) a continuous aqueous phase (b) a source of borate ions, (c) a source of ions selected from the group consisting of alkali metal ions, alkaline earth metal ions and ammonium ions and (d) a source of at least one sugar selected from the group consisting of monosaccharides and oligosaceharides having from 2 to 4 saceharide groups.
 34. A concentrate comprising a source of borate ions, a source of ions selected from the group consisting of alkali metal ions, alkaline earth metal ions and ammonium ions and a source of at least one sugar selected from the group consisting of monosaccharides and oligosaccharides having from 2 to 4 saceharide groups dissolved or dispersed in an aqueous phase.
 35. A concentrate as claimed in claim 34 wherein the concentration of borate, expressed as B₂O₃, is in the range 10 to 250 lb/bbl, the concentration of alkali metal salt, alkaline earth metal salt or ammonium salt is in the range 10 to 150 lb/bbl and the concentration of sugar is in the range 10 to 300 lb/bbl.
 36. A concentrate as claimed in claim 34 wherein the source of borate ions is as defined in claim
 3. 37. A concentrate as claimed in claim 34 wherein the source of the alkali metal ions, alkaline earth metal ions or the ammonium ions is as defined in claim
 6. 38. A concentrate as claimed in claim 34 wherein the source of the sugar is as defined in claim
 10. 39. Use of a composition according to claim 33 as a drilling fluid. 